Anomalous splitting of the F1u(-->3Fu) vibrations in single-crystal C60 below the orientational-ordering transition.

Abstract

The reflectance of undoped single-crystal ${\mathrm{C}}_{60}$ has been measured from 100 to 1600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at high resolution above and below the orientational-ordering transition. All of the ${\mathit{F}}_{1\mathit{u}}$ modes allowed in the free molecule are observed in the solid at \ensuremath{\approxeq}526, 576, 1183, and 1430 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Below the orientational-ordering transition, all the lines split into quartets, with the exception of the 576 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ line, which narrows and remains unsplit. The appearance of quartets is attributed to the possibility of triplet splitting of the ${\mathit{F}}_{1\mathit{u}}$(->3${\mathit{F}}_{\mathit{u}}$) modes due to the local crystal field in the low-temperature phase, as well as the perturbation of the local crystal field due to the merohedral disorder, that may lead to further line splitting.